JP6171851B2 - Apparatus row for seamless steel pipe production and method for producing high-strength stainless steel seamless steel pipe for oil wells using the same - Google Patents

Description

  The present invention relates to the production of seamless steel pipes, and more particularly to an apparatus row suitable for producing seamless steel pipes and a method for producing a high-strength stainless steel seamless pipe for oil wells having excellent low temperature toughness using the apparatus row.

  In recent years, from the viewpoint of soaring energy prices such as crude oil due to an increase in global energy consumption, and the depletion of petroleum resources, deep oil fields (deep oil fields) and hydrogen sulfide that have not been previously excluded Energy resources are being actively developed in oil fields and gas fields in severe corrosive environments under a so-called sour environment, and in oil fields and gas fields in the extreme north of severe weather environments. The oil well steel pipe used in such an environment is required to have a material having high strength and excellent corrosion resistance (sour resistance) and excellent low temperature toughness.

Conventionally, 13% Cr martensitic stainless steel pipes are often used as oil well pipes used for mining in environmental oil fields and gas fields containing carbon dioxide CO ₂ , chlorine ions Cl ^{− and the} like. Furthermore, recently, the use of improved 13Cr martensitic stainless steels with a reduced content of 13Cr martensitic stainless steel and increased Ni, Mo, etc. has been expanded.
For example, Patent Document 1 describes a method for producing an improved martensitic stainless steel (steel plate) in which the corrosion resistance of 13% Cr martensitic stainless steel is improved. In the technique described in Patent Document 1, the composition of martensitic stainless steel containing 10 to 15% Cr by weight is limited to 0.005 to 0.05% for C, Ni: 4.0% or more, Cu: 0.5 to A steel with a composition with 3% added, Mo added 1.0 to 3% and Nieq adjusted to -10 or higher is hot-worked and allowed to cool naturally to room temperature. Heat treatment is performed below the temperature at which the fraction reaches 80%, heat treatment is performed at a temperature at which the austenite fraction reaches 60%, and the structure is composed of a tempered martensite phase, a martensite phase, and a retained austenite phase. The martensitic stainless steel has a structure in which the total fraction of the martensite phase is 60 to 90%. As a result, the corrosion resistance and sulfide stress corrosion cracking resistance in a wet carbon dioxide environment and a wet hydrogen sulfide environment are improved.

Patent Document 2 describes a method for producing a high-strength stainless steel pipe for oil wells having excellent corrosion resistance. In the technique described in Patent Document 2, in mass%, C: 0.005 to 0.050%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, Cr: 15.5 to 18%, Ni: 1.5 to 5%, Mo : 1 to 3.5%, V: 0.02 to 0.20%, N: 0.01 to 0.15%, O: 0.006% or less, Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 and Cr + Mo + 0.3Si-43.5C-0.4Mn -Ni-0.3Cu-9N≥11.5 steel pipe material is heated, piped by hot working, and then piped and then cooled to room temperature at a cooling rate equal to or higher than that of air cooling to seamlessly pass through the specified dimensions. By performing a quenching-tempering treatment in which the steel pipe is then reheated to a temperature of 850 ° C or higher, cooled to 100 ° C or lower at a cooling rate of air cooling or higher, and then heated to a temperature of 700 ° C or lower. A high-strength stainless steel pipe for oil wells having a structure containing a ferrite phase of 10 to 60% by volume and the balance being a martensite phase and a yield strength of 654 MPa or more can be obtained. To have. Accordingly, high strength, CO ₂ and Cl ^- containing, has sufficient corrosion resistance even at a high temperature severe corrosive environments up to 230 ° C., moreover absorbed energy at -40 ℃ Charpy impact test is more than 50J The steel pipe has high toughness.

Japanese Patent Laid-Open No. 10-1755 Japanese Patent No. 5109222

  Steel pipes of various thicknesses and diameters are required for oil well seamless steel pipes. In particular, in the manufacture of thick-walled seamless steel pipes, as the wall thickness increases, the normal hot working method makes it difficult to apply processing strain to the center of the wall thickness, and the structure of the wall thickness center part becomes difficult. It tends to be coarse. Therefore, compared with a thin steel pipe, in a thick steel pipe, the toughness of the thickness center part tends to decrease. The techniques described in Patent Documents 1 and 2 are intended for steel pipes with a thickness of up to 12.7 mm, and Patent Documents 1 and 2 refer to the improvement in low-temperature toughness of thick-walled seamless steel pipes. Absent.

  In view of the state of the prior art, it is an object of the present invention to provide a device array for manufacturing seamless steel pipes that can inexpensively manufacture a thick stainless steel seamless steel pipe having excellent low temperature toughness. In addition, the present invention combines these devices with high strength exceeding yield strength: 654 MPa, excellent corrosion resistance under a high temperature corrosion environment, and excellent low temperature toughness at the center of the thickness. It is an object of the present invention to provide a method for producing a high-strength thick stainless steel seamless steel pipe for oil wells, which can provide a high-strength thick stainless steel seamless steel pipe for oil wells. The term “thick-walled seamless steel pipe” as used herein refers to a seamless steel pipe having a wall thickness exceeding 13 mm and not more than about 100 mm.

In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting the toughness of the thick-walled stainless steel seamless steel tube wall thickness central portion. As a result, it came to mind that the most effective method for improving toughness is the refinement of the structure.
In order to refine the structure of the thick-walled martensitic stainless steel seamless tube, the hollow material after piercing and rolling should have a temperature range of 600 ° C or higher and a temperature range of 50 ° C or higher. Cooling at an average cooling rate of 1.0 ° C / s or higher, which is a cooling rate higher than air cooling, and further reducing the thickness or processing such as forming, the structure becomes finer and the wall thickness: 13 mm thick stainless steel seamless The knowledge that low temperature toughness is remarkably improved also at the thickness center position of the steel pipe was obtained.

First, the experimental results that were the basis of the present invention conducted by the present inventors will be described.
0.017% C-0.19% Si-0.26% Mn-0.01% P-0.002% S-16.6% Cr-3.5% Ni-1.6% Mo-0.047% V-0.047% N-0.01% Al-balance Fe A test material was collected from a martensitic stainless steel seamless pipe for an oil well having a composition consisting of: The collected test material was heated to a heating temperature of 1250 ° C. and held for a certain time (60 min), and then cooled at various cooling rates up to a cooling stop temperature in the range of 1200 to 600 ° C. which is a hot working temperature range. After completion of cooling, the test material was immediately quenched to freeze the tissue.

Subsequently, the obtained test piece was polished and corroded (corrosion solution: Villera solution), and the structure was observed, and the area ratios of the martensite phase and the ferrite phase were measured. The martensite phase is a phase in which the austenite phase present at the cooling stop temperature is transformed upon rapid cooling. The obtained results are shown in FIG. 2 in relation to the average cooling rate and the ferrite amount (area ratio) at the cooling stop temperature.
From FIG. 2, regardless of the cooling stop temperature, the temperature range from the heating temperature to the cooling stop temperature (hot working temperature) is cooled at 0.5 ° C./s by cooling at an average cooling rate of 1.0 ° C./s or more. It can be seen that the ferrite phase fraction is higher than that of the case. The cooling at an average cooling rate of 0.5 ° C./s is a cooling simulating air cooling (equivalent to air cooling) and can be said to be cooling in a state close to equilibrium.

  That is, in the martensitic stainless steel having the composition described above, the ferrite phase fraction is usually high in the heating temperature range, and when cooling from the heating temperature to a cooling rate of about air cooling, the ferrite decreases with decreasing temperature. The phase decreases and the fraction of the austenite phase increases. However, accelerated cooling of the temperature range from the heating temperature to the hot working temperature (cooling stop temperature) at an average cooling rate of 1.0 ° C / s or more delays the precipitation of the austenite phase and increases the ferrite phase from the equilibrium state. It remains and a non-equilibrium phase distribution (structure) is obtained.

The present inventors have found that the structure can be made finer by subjecting the material having such a non-equilibrium structure to processing (rolling). This is because, if strain is added to the ferrite grains that exist in non-equilibrium, a large number of α → γ transformation nucleation sites can be generated, and as a result, the austenite grains generated after the transformation become finer and the low temperature toughness is improved. It is done.
And in order to make it possible to produce a stainless steel seamless steel pipe for oil wells having excellent low-temperature toughness using the above-mentioned phenomenon, the present inventors have used a device row, a heating device, a piercing and rolling device, and a rolling device. It is found that it is important to use a device row in which a cooling device is arranged between the heating device and the piercing and rolling device or between the piercing and rolling device and the rolling device, from the conventional device row in which the above are arranged in this order. did.

The present invention has been completed on the basis of such findings and further studies. That is, the gist of the present invention is as follows.
(1) A heating device for heating a steel material, a piercing and rolling device for subjecting the heated steel material to piercing and rolling to form a hollow material, and a rolling device for processing the hollow material to obtain a seamless steel pipe having a predetermined shape. Are arranged in this order, in an apparatus row for seamless steel pipe production, between the heating device and the piercing-rolling device or between the piercing-rolling device and the rolling device, the outer surface of the material to be cooled An apparatus row for manufacturing a thick-walled stainless steel seamless pipe comprising a cooling apparatus having a cooling capacity of 1.0 ° C./s or more at an average cooling rate at a position .

(2) Oite to (1), the delivery side of the rolling apparatus, the thick stainless seamless steel manufacturing equipment train, characterized by disposing the heat insulating device.
(3) (1) or (2) a method of manufacturing a thick-walled stainless seamless steel pipe using a thick stainless seamless steel manufacturing equipment train according, after heating the steel material in the heating device, wherein A hollow material is formed by piercing and rolling with a piercing and rolling device, and the hollow material is further cooled with the cooling device, then processed with the rolling device, or further subjected to a process of passing through the heat retaining device after the processing. Furthermore, when a heat treatment consisting of quenching and tempering treatment or tempering treatment is performed to make a thick stainless steel seamless steel pipe of a predetermined size, the steel material is in mass%, C: 0.050% or less, Si: 0.50% or less , Mn: 0.20-1.80%, Cr: 15.5-18.0%, Ni: 1.5-5.0%, Mo: 1.0-3.5%, V: 0.02-0.20%, N: 0.01-0.15%, O: 0.006% or less A steel material having a composition comprising the balance Fe and inevitable impurities, and the heating In addition, the heat treatment is performed at a temperature of 600 ° C. or higher and less than the melting point of the steel material, and after the piercing and rolling, the surface temperature of the hollow material before cooling with the cooling device is set as a cooling start temperature, and the cooling is performed. At the surface temperature, the temperature difference from the cooling start temperature is at least 50 ° C. or more, and the cooling is performed at an average cooling rate of 1.0 ° C./s or more at the outer surface temperature to the cooling stop temperature at which the temperature is 600 ° C. or more . The above-mentioned thick stainless steel seamless steel pipe has a high strength with a yield strength exceeding 654 MPa in the tube axis direction, and a Charpy impact test temperature at the center of the wall thickness: absorbed energy in the tube axis direction at −40 ° C. is 50 J or more A method for producing a high-strength thick stainless steel seamless steel pipe for oil wells having excellent low-temperature toughness, characterized by being a high-strength thick-walled stainless steel seamless steel pipe having both excellent low-temperature toughness.

( 4 ) In ( 3 ), the process of passing through the heat retaining device after the processing is a process of adjusting the cooling so that the average cooling rate is 20 ° C./s or less. Manufacturing method for thick-walled stainless steel seamless steel pipe.
( 5 ) In ( 3 ) or ( 4 ), in addition to the above-mentioned composition, the following groups A to D: Group A: Al: 0.002 to 0.050%
Group B: Cu: 3.5% or less,
Group C: Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less selected from 0.01% or less,
D group: Ca: 0.01% or less, REM: High strength stainless steel for oil wells containing one or more groups selected from one or two selected from 0.01% or less A method for producing seamless steel pipes.

  According to the present invention, a thick high-strength stainless steel seamless pipe excellent in low-temperature toughness can be easily manufactured, and an industrially remarkable effect is achieved. In addition, according to the present invention, the steel pipe structure can be refined to the center with a relatively small amount of processing, and even in thick-walled seamless steel pipes where the amount of processing at the center of the wall thickness cannot be increased, There is an effect that the improvement can be achieved.

It is explanatory drawing which shows typically an example of the apparatus row | line | column for this invention seamless steel pipe manufacture. It is a graph which shows the relationship between the average cooling rate before hot processing, and the ferrite content in cooling stop temperature.

  The apparatus row for manufacturing the seamless steel pipe of the present invention is an apparatus row that can be processed into a seamless steel pipe after cooling the heated steel material within an appropriate temperature range. An example of the apparatus row | line | column for this invention seamless steel pipe manufacture is shown in FIG. The apparatus row | line | column for this invention seamless steel pipe manufacture arrange | positions the heating apparatus 1, the piercing-rolling apparatus 2, the cooling apparatus 4, and the rolling apparatus 3 in this order, or (b) the heating apparatus 1, the cooling apparatus 4, and piercing | punching. The rolling device 2 and the rolling device 3 are arranged in this order to form a device row.

The heating apparatus 1 used in the present invention can heat steel materials such as round cast pieces and round steel pieces to a predetermined temperature. For example, a conventional heating furnace such as a rotary hearth type heating furnace or a walking beam type heating furnace can be used. Is also applicable. Alternatively, an induction heating type heating furnace may be used.
Further, the piercing and rolling apparatus 2 used in the present invention may be any piercing and rolling apparatus that can perform piercing and rolling on a heated steel material to form a hollow material. For example, Mannesmann inclined piercing using a barrel-shaped roll or the like. Any generally known piercing and rolling apparatus such as a machine or a hot extrusion piercing machine can be applied.

  The rolling device 3 used in the present invention may be any device that can process a hollow material into a seamless steel pipe having a predetermined shape. For example, an elongator 31 or a perforated hollow element can be used depending on the purpose. A rolling mill arranged in the order of a plug mill 32 for extending the tube thinly and long, a reeler (not shown) for smoothing the inner and outer surfaces of the tube, a sizer 33 for adjusting the tube to a predetermined size, or a mandrel having a hollow tube as a steel tube of a predetermined size Any generally known rolling device such as a mill (not shown), a rolling device provided with a reducer (not shown) that slightly reduces the outer diameter and adjusts the wall thickness can be applied. In addition, it is preferable to use an elongator or mandrel mill that can take a large amount of processing.

  The cooling device 4 used in the present invention is installed between the heating device 1 and the piercing and rolling device 2 or between the piercing and rolling device 2 and the rolling device 3 in order to obtain a non-equilibrium phase distribution. . The type of the cooling device used in the present invention is not particularly limited as long as it is a device capable of cooling a heated steel material (cooled material) at a desired cooling rate or higher. As a cooling device that can secure a desired cooling rate relatively easily, cooling water, compressed air, or mist is injected or supplied to the outer surface of a heated steel material or hollow material that is a material to be cooled. It is preferable to use a cooling system.

  The cooling device used in the present invention obtains an average cooling rate of at least 1.0 ° C./s or more at the outer surface position of the material to be cooled in order to obtain a non-equilibrium phase distribution when manufacturing a steel pipe having a stainless steel composition. It is necessary to provide a device having a cooling capacity that can be used. If the cooling capacity of the cooling device is insufficient and cooling can only be performed slower than the average cooling rate described above, a phase distribution in a non-equilibrium state cannot be obtained, and the microstructure can be refined even after subsequent processing. Disappear. The upper limit of the cooling rate is not particularly limited, but is preferably 30 ° C./s from the viewpoint of preventing cracking and bending due to thermal stress.

  In the present invention, it is preferable to use a device row in which a heat retaining device (not shown) is provided on the exit side of the rolling device 3. In this invention, in order to slow down the cooling rate after a rolling process, a heat retention apparatus is arrange | positioned. In the case of stainless steel pipe, if the cooling is too fast after processing, the nonequilibrium ferrite phase is cooled without causing the α → γ transformation, and fine austenite grains cannot be formed, and the desired refinement of the steel pipe structure can be achieved. Disappear. Note that it is sufficient for the heat retaining device to have a heat retaining ability that can be adjusted to a cooling rate of at least about 20 ° C./s at the surface position of the material to be cooled.

Next, a method for producing a thick high-strength stainless steel seamless steel pipe for oil wells that is high in strength, excellent in corrosion resistance, and excellent in low-temperature toughness will be described using the above-described apparatus for producing a seamless steel pipe of the present invention.
After the steel material is heated by the heating device, the steel material is pierced and rolled by a piercing and rolling device to form a hollow material, and the hollow material is further cooled by a cooling device, and then immediately processed by the rolling device, or after the processing, a further heat retaining device. To give a seamless steel pipe of a predetermined size.

The steel materials used are mass%, C: 0.050% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, Mo: 1.0 to 3.5%, V: 0.02 to 0.20%, N: 0.01 to 0.15%, O: 0.006% or less, and a composition comprising the balance Fe and inevitable impurities.
First, the reasons for limiting the composition of the steel material will be described. Unless otherwise specified, mass% is simply expressed as%.

C: 0.050% or less
C is an important element related to the strength of martensitic stainless steel. In the present invention, C is preferably contained in an amount of 0.005% or more in order to ensure a desired strength. On the other hand, if the content exceeds 0.050%, sensitization during tempering due to Ni inclusion increases. From the viewpoint of corrosion resistance, it is desirable that C is small. For these reasons, C is limited to 0.050% or less. In addition, Preferably it is 0.030 to 0.050%.

Si: 0.50% or less
Si is an element that acts as a deoxidizer, and it is desirable to contain 0.05% or more. On the other hand, if the content exceeds 0.50%, the corrosion resistance is lowered and the hot workability is also lowered. For this reason, Si was limited to 0.50% or less. In addition, Preferably it is 0.10 to 0.30%.
Mn: 0.20 to 1.80%
Mn is an element having an action of increasing the strength, and in order to obtain such an effect, it needs to be contained in an amount of 0.20% or more. On the other hand, if the content exceeds 1.80%, the toughness is adversely affected. For this reason, Mn was limited to 0.20 to 1.80%. In addition, Preferably it is 0.2 to 1.0%.

Cr: 15.5-18.0%
Cr is an element that has a function of forming a protective film and improving the corrosion resistance, and further increasing the strength of the steel by solid solution. In order to obtain such an effect, the content of 15.5% or more is required. On the other hand, if the content exceeds 18.0%, the hot workability is lowered and the strength is further lowered. For this reason, Cr was limited to 15.5-18.0%. In addition, Preferably it is 16.5-18.0%.

Ni: 1.5-5.0%
Ni is an element that has an action of strengthening the protective film and improving the corrosion resistance, and further increasing the strength of the steel by solid solution and further improving the toughness. Such an effect is recognized when the content is 1.5% or more. On the other hand, if the content exceeds 5.0%, the stability of the martensite phase decreases and the strength decreases. For this reason, Ni was limited to 1.5 to 5.0%. In addition, Preferably it is 2.5 to 4.5%.

Mo: 1.0-3.5%
Mo is an element that increases resistance to pitting corrosion caused by Cl ⁻ , and needs to be contained in an amount of 1.0% or more. On the other hand, if the content exceeds 3.5%, the strength decreases and the material cost increases. For this reason, Mo was limited to 1.0 to 3.5%. In addition, Preferably it is 2 to 3.5%.
V: 0.02 to 0.20%
V is an element that increases strength and improves corrosion resistance. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, if the content exceeds 0.20%, toughness decreases. For this reason, V was limited to 0.02 to 0.20%. In addition, Preferably it is 0.02 to 0.08%.

N: 0.01-0.15%
N is an element that remarkably improves the pitting corrosion resistance. In order to obtain such an effect, N is required to be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.15%, various nitrides are formed and the toughness is lowered. In addition, Preferably it is 0.02 to 0.08%.
O: 0.006% or less
O exists as an oxide in steel and adversely affects various properties. For this reason, it is desirable to reduce as much as possible. In particular, when O is contained in a large amount exceeding 0.006%, the hot workability, toughness and corrosion resistance are remarkably deteriorated. For this reason, O was limited to 0.006% or less.

The above-mentioned components are basic components. In addition to the basic components, the following group A to group D: Group A: Al: 0.002 to 0.050%,
Group B: Cu: 3.5% or less,
Group C: Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less selected from 0.01% or less,
Group D: Ca: 0.01% or less, REM: One or more groups selected from one or two selected from 0.01% or less can be contained.

Group A: Al: 0.002 to 0.050%,
Group A: Al is an element that acts as a deoxidizer. In order to obtain such an effect, it is preferably contained in an amount of 0.002% or more, but if it exceeds 0.050%, the toughness is adversely affected. For this reason, when it contains, it is preferable to limit to A group: Al: 0.002-0.050%. More preferably, it is 0.03% or less. In the case where Al is not added, Al: less than 0.002% is allowed as an inevitable impurity.

Group B: Cu: 3.5% or less Group B: Cu strengthens the protective film, suppresses the penetration of hydrogen into the steel, and improves the resistance to sulfide stress corrosion cracking. In order to obtain such an effect, it is desirable to contain 0.5% or more, while the content exceeding 3.5% leads to precipitation of CuS grain boundaries and decreases hot workability. For this reason, when it contains, it is preferable to limit B group: Cu to 3.5% or less. In addition, More preferably, it is 0.8 to 2.5%.

Group C: Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less selected from Group C: Nb, Ti , Zr, W, and B are elements that increase the strength, and can be selected and contained as necessary. Such effects are recognized when Nb: 0.03% or more, Ti: 0.03% or more, Zr: 0.03% or more, W: 0.2% or more, B: 0.0005% or more. On the other hand, inclusions exceeding Nb: 0.2%, Ti: 0.3%, Zr: 0.2%, W: 3.0%, and B: 0.01% respectively reduce toughness. For this reason, when it contains, it is preferable to limit to Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less, respectively.

Group D: Ca: 0.01% or less, REM: One or two types selected from 0.01% or less Group D: Ca, REM has the effect of spheroidizing the form of sulfide inclusions, and intervenes It has the effect of reducing the lattice strain of the matrix around the object and reducing the hydrogen trapping ability of the inclusions, and it can contain one or two kinds as required. In order to obtain such an effect, it is desirable to contain Ca: 0.0005% or more and REM: 0.001% or more. However, if it contains more than Ca: 0.01% and REM: 0.01%, the corrosion resistance decreases. For this reason, when it contains, it is preferable to limit Ca to 0.01% or less and REM to 0.01% or less.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include P: 0.03% or less and S: 0.005% or less.
The method for producing a steel material having the above composition need not be particularly limited. Using a conventional smelting furnace such as a converter or electric furnace, the molten steel having the composition described above is melted, and a slab (round slab) is obtained by a conventional casting method such as a continuous casting method. It is preferable to use a raw material. In addition, it is good also as a steel raw material as a steel slab of a predetermined dimension by hot-rolling a slab. Moreover, it is set as a steel slab by the ingot-making-slab rolling method, and there is no problem as a steel raw material.

First, a steel material having the above-described composition is charged into a heating device and heated to a temperature in the range of heating temperature: 600 ° C. or higher and lower than the melting point.
Heating temperature: 600 ° C. or higher and lower than the melting point When the heating temperature is lower than 600 ° C., the structure is a single phase, and the structure cannot be refined using transformation. On the other hand, processing cannot be performed above the melting point. For this reason, the heating temperature of the steel material was limited to a temperature of 600 ° C. or higher and lower than the melting point. The temperature is preferably 1000 to 1300 ° C. from the viewpoint that the deformation resistance is small and the processing is easy and the temperature difference during cooling can be increased. More preferably, it is 1100-1300 degreeC.

The heated steel material is then subjected to piercing and rolling with a piercing and rolling device to form a hollow material.
The piercing and rolling conditions are not particularly limited as long as the steel material can be a predetermined hollow material, and it is preferable to use the usual piercing and rolling.
Next, the obtained hollow material is cooled by a cooling device.
Cooling is accelerated by cooling at an average cooling rate of 1.0 ° C / s or higher at the temperature of the outer surface position of the hollow material to a cooling stop temperature at which the temperature difference from the cooling start temperature is at least 50 ° C and 600 ° C or higher. Let it be a cooling process. The cooling start temperature is the thickness center temperature of the hollow material before the start of cooling, and is preferably 600 ° C. or higher in the present invention. When the cooling start temperature is less than 600 ° C., the fine structure effect by the subsequent processing cannot be expected.

Cooling temperature range: 50 ° C. or more The cooling temperature range, that is, the temperature difference between the cooling start temperature and the cooling stop temperature is at least 50 ° C. at the surface temperature. When the cooling temperature range is less than 50 ° C., it becomes impossible to secure a remarkable non-equilibrium phase fraction, and the desired structure refinement cannot be achieved by subsequent processing. For this reason, the temperature range of cooling was limited to 50 ° C. or higher. The larger the cooling temperature range, the easier it is to secure a non-equilibrium phase fraction. In addition, Preferably it is 100 degreeC or more.

Cooling stop temperature: 600 ° C or higher Cooling stop temperature shall be 600 ° C or higher. When the cooling stop temperature is less than 600 ° C., the diffusion of elements is delayed, the phase transformation (α → γ transformation) by the subsequent processing is delayed, and the microstructure effect by the desired processing cannot be expected. For this reason, the cooling stop temperature was limited to 600 ° C. or higher. In addition, Preferably it is 700 degreeC or more. Even if the temperature at the time of cooling stop is less than 600 ° C., the effect of refining the structure is exhibited when the heat generation due to double heat or subsequent hot processing is 600 ° C. or higher.

Average cooling rate: 1.0 ° C./s or more If the average cooling rate of cooling is less than 0.3 ° C./s, it is impossible to secure a phase fraction in a non-equilibrium state, and a desired microstructure cannot be achieved by subsequent processing. For this reason, the cooling rate of cooling is limited to 1.0 ° C./s or more. The upper limit of the cooling rate is determined by the capacity of the cooling device and is not particularly limited, but is preferably 30 ° C./s or less from the viewpoint of preventing cracking and bending due to thermal stress. More preferably, it is 3 to 10 ° C./s.

The cooled hollow material is processed by a rolling device to be a seamless steel pipe having a predetermined size. In addition, it is preferable that the time until the processing is performed is within 600 s after the cooling is completed. If the time from the end of cooling to the start of processing exceeds 600 s, the ferrite phase transforms into an austenite phase, making it difficult to ensure a non-equilibrium state.
The cooling rate after processing is not particularly limited. However, when cooling is performed at an average cooling rate exceeding 20 ° C./s at the wall thickness center temperature, the heat retaining temperature provided on the outlet side of the rolling apparatus. It is preferable to charge the apparatus and adjust the average cooling rate to 20 ° C./s or less. If the cooling after processing exceeds 20 ° C / s and becomes too fast, the precipitation of the austenite phase due to the α → γ transformation is delayed, the austenite phase is cooled without precipitation, the processed structure is frozen, and the desired structure It becomes impossible to achieve miniaturization.

  In the above, the case where the cooling apparatus used the apparatus row | line | column arrange | positioned between the piercing-rolling apparatus and the rolling apparatus was demonstrated. In the present invention, the same effect can be expected even when a cooling device using a device row arranged between a heating device and a piercing and rolling device is used. This is because, in the present invention, it is confirmed that there is an effect in both piercing and rolling, and the influence of the processing form of the processing apparatus is small.

  When using a device array in which the cooling device is arranged between the heating device and the piercing and rolling device, the cooling stop temperature is set according to the steel type so that the temperature range for cooling is equal to or higher than the temperature at which piercing and rolling can be performed. Need to be set. If it is the composition range of the steel raw material used by this invention, it is preferable that a cooling stop temperature shall be 1000 degreeC or more. When the cooling stop temperature is less than 600 ° C., the deformation resistance becomes too high and piercing and rolling becomes difficult. For this reason, in this case, the cooling stop temperature is preferably limited to 600 ° C. or higher. Further, when cooling the heated steel material, in order to ensure a non-equilibrium phase fraction, the cooling rate at the outer surface position should be, on average, a cooling rate of 1.0 ° C./s or more. preferable.

The seamless steel pipe obtained by the above-described manufacturing method is a steel pipe having the above-described composition and a structure including a martensite phase as a main phase, a ferrite phase, and a residual austenite phase. The “main phase” here refers to the most common phase. The residual austenite phase is preferably 20% or less in terms of area ratio. A steel pipe having such a structure has a high strength exceeding yield strength: 654 MPa, an excellent low temperature toughness with an absorbed energy at a Charpy impact test at the center of the wall thickness of −40 ° C. of 50 J or more. It contains carbon dioxide and becomes a steel pipe with excellent corrosion resistance in a severe corrosive environment at a high temperature of 230 ° C.

  Next, the present invention will be further described based on examples.

A steel material having the composition shown in Table 1 was used as a starting material. In these steel materials, molten steel melted in a converter is made into a slab by a continuous casting method, and the slab is made into a round steel piece (230 mmφ) having a composition shown in Table 1 by forming and rolling. These steel materials were used to make thick-walled seamless steel pipes (outer diameter 273 mmφ x wall thickness 32 mm).
These steel materials are charged into the heating device 1 of the apparatus row shown in FIG. 1 (a), heated to the heating temperature shown in Table 2 and held for a certain time (60min), and then a Mannesmann piercing and rolling apparatus with a barrel roll. 2 is used for piercing and rolling to form a hollow material (thickness: about 50 mm), and using a cooling device 4 that uses cooling water by spraying as a refrigerant, at the average cooling rate shown in Table 2, up to the cooling stop temperature shown in Table 2 It was cooled and immediately rolled with a rolling reduction device 3 in which an elongator, a plug mill, a reeler, and a sizer were sequentially arranged at the cumulative reduction shown in Table 2 to obtain a seamless steel pipe (outer diameter 273 mmφ × thickness 25-50 mm). In addition, after rolling, it was allowed to cool (0.1 to 1.5 ° C./s). The resulting thick-walled seamless steel pipe was further subjected to heat treatment (quenching / tempering treatment or tempering treatment).

Test pieces were collected from the resulting thick-walled seamless steel pipe and subjected to structure observation, tensile test, and impact test. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained steel pipe, and a cross section (C cross section) perpendicular to the longitudinal direction of the pipe is polished and corroded (corrosive liquid: Villera liquid), and an optical microscope (magnification: 100 times) or a scanning electron microscope (magnification: 1000 times), the tissue was observed, imaged, and image analysis was used to measure the type and fraction of the tissue. Note that the number of phase boundaries intersecting with the unit length straight line was measured from the structure photograph, and used as a crystal grain size index and a refinement index. In addition, the number of phase boundaries per unit length was shown as a ratio to the reference value, with the obtained value being the value of steel pipe No. 5 as the reference (1.00).

(2) Tensile test From the obtained steel pipe, take a round bar tensile test piece (parallel part 6mmφ x GL20mm) so that the tube axis direction is the tensile direction, conduct a tensile test, yield strength YS, tensile We asked for strength TS. The yield strength was 0.2% elongation.
(3) Impact test V-notch test specimens are collected from the center position of the thickness of the obtained steel pipe so that the pipe axis direction is the longitudinal direction of the specimen, and Charpy impact tests are conducted in accordance with the provisions of JIS Z 2242. The test was carried out to measure the absorbed energy at a test temperature of −40 ° C. to evaluate toughness. The number of test pieces was three each, and the average value thereof was taken as the absorbed energy of the steel pipe.

  The results obtained are shown in Table 3.

In all of the examples of the present invention, the structure can be refined even at the thickness center position of the thick wall, and although the yield strength is higher than 654 MPa, the Charpy impact test temperature is −40 ° C. Absorbed energy is 50J or more, and the toughness is remarkably improved. In addition, the toughness is remarkably improved even in the present invention example (steel pipe No. 11) having a low processing amount (rolling rate) of 0%. On the other hand, the comparative example which is out of the scope of the present invention does not ensure the desired high strength, or the structure cannot be refined, and the desired high toughness cannot be ensured.

1 Heating device 2 Punching and rolling device 3 Rolling device 4 Cooling device
31 Elongator
32 Plug mill
33 Sizing mill (sizer)

Claims (5)

A heating device for heating a steel material, a piercing and rolling device for subjecting the heated steel material to piercing and rolling to form a hollow material, and a rolling device for processing the hollow material to obtain a seamless steel pipe having a predetermined shape in this order. In the apparatus row for seamless steel pipe production, the steel material is a steel material having a martensitic stainless steel composition, and the heating device heats the steel material to a heating temperature of 600 ° C. or higher and lower than the melting point. A heating device having a heating capability capable of being heated, and between the heating device and the piercing and rolling device, on the exit side of the heating device and on the entry side of the piercing and rolling device, or the piercing and rolling device and the A temperature range from the cooling start temperature to the cooling stop temperature at which the temperature difference from the cooling start temperature is at least 50 ° C. or more and 600 ° C. or more is provided between the rolling device and the exit side of the piercing and rolling device and on the entry side of the rolling device. , of the outer surface position of the coolant Thick, characterized in that have a cooling ability 1.0 ° C. / s or more average cooling rate is obtained, formed by disposing can be Ru cooler be organized in a non-equilibrium state tissue of a coolant Equipment line for stainless steel seamless steel pipe manufacturing.   The apparatus for manufacturing a thick-walled stainless steel seamless steel pipe according to claim 1, wherein a heat retaining device is disposed on the outlet side of the rolling device. A method for producing a thick stainless steel seamless steel pipe using the apparatus for producing a thick stainless steel seamless steel pipe according to claim 1 or 2, wherein a steel material is heated by the heating device and then punched by the piercing and rolling device. A hollow material is formed by rolling, and the hollow material is further cooled by the cooling device, then processed by the rolling device, or further processed by passing through the heat retaining device after the processing, and further quenched and hardened. When performing heat treatment consisting of tempering treatment or tempering treatment to make a thick stainless steel seamless steel pipe of a predetermined size,
The steel material in mass%,
C: 0.050% or less, Si: 0.50% or less,
Mn: 0.20-1.80%, Cr: 15.5-18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, N: 0.01 to 0.15%,
O: A steel material containing 0.006% or less and having the balance Fe and inevitable impurities,
The heating is performed at a temperature of 600 ° C. or higher and lower than the melting point, and after the piercing and rolling, the surface temperature of the hollow material before cooling with the cooling device is set as a cooling start temperature, and the cooling is performed. At the surface temperature, the temperature difference from the cooling start temperature is at least 50 ° C. or more, and the cooling is performed at an outer surface temperature at an average cooling rate of 1.0 ° C./s or more to a cooling stop temperature that is 600 ° C. or more. Thick-walled stainless steel seamless pipe has a high strength with a yield strength exceeding 654 MPa in the tube axis direction, and a Charpy impact test temperature at the center of the wall thickness: Absorbed energy in the tube axis direction at -40 ° C is 50 J or more. A method for producing a high-strength, thick stainless steel seamless steel pipe for oil wells having excellent low-temperature toughness, characterized by being a high-strength, thick-walled stainless steel seamless steel pipe having both excellent low-temperature toughness.   The high-strength thick wall for oil wells according to claim 3, wherein the process of passing through the heat retaining device after the processing is adjusted so that the average cooling rate is 20 ° C / s or less. Manufacturing method of stainless steel seamless steel pipe. The high strength thickness for oil wells according to claim 3 or 4, further comprising one group or two or more groups selected from the following groups A to D in mass% in addition to the composition. Manufacturing method of meat stainless steel seamless steel pipe.
Group A: Al: 0.002 to 0.050%,
Group B: Cu: 3.5% or less,
Group C: Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less selected from 0.01% or less,
Group D: Ca: 0.01% or less, REM: One or two selected from 0.01% or less

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